Amphibious vehicles normally have at least the driver's seat adjustably mounted to the floor of the vehicle by means of a track comprising a pair of rails attached directly to the floor. The seat engages with the track by means of rollers and a latch is provided to hold the seat at a particular position by latching onto the rail. This type of arrangement is also used for normal road vehicles. A problem with amphibious vehicles is that when they are open topped, water splashes into the vehicle and may slop around on the floor; and sand may also blow into the vehicle. The combination of sand and water in and around the seat track may well cause corrosion and/or jamming of the seat on its track. This would be the situation if a seat mounting of the sort shown in U.S. Pat. No. 4,889,379 (Aso) were fitted to an amphibious vehicle.
A further problem is that steering an amphibious vehicle in its marine mode requires the driver to sit higher than the position best suited for road use. This is even more the case where the vehicle is designed to plane, as a favourable static weight distribution to allow planning is tail heavy, so that the vehicle sits nose up when afloat; and even more nose up when planing. Passengers may also appreciate the option of a higher seating position on water, for improved visibility, and for the exhilaration of the wind in their faces. However, a lower seating position on land is preferred to allow better protection from wind and in vehicle collisions; also greater comfort at the higher speeds attained on roads than on water.
If a higher seating position is offered for marine use, the centre of gravity of the vehicle occupants will be raised, compared to their centre of gravity on road. If conventional seat mountings are used, with dual seat tracks below the seat and inboard of the edges of the seat, the vertical distance from the seat mounts to the occupant's torso is much greater than the lateral distance between the seat tracks. The seats may oscillate as the occupants shift from side to side according to the movement of the vehicle, causing the occupants to feel insecure. The high bending moments applied to seat and mounting components may also cause rapid wear.
The issue of perceived seat stability is particularly germane to a centre mounted seat, as there is no practical possibility in this case of the occupant restraining himself or herself against the vehicle interior trim on door, side panel, or centre console, should the seat mounting be considered to have lateral movement beyond the occupant's envelope of comfort and security.